In recent years, materials that include polymeric resins have replaced metal components for many aerospace applications. For example, polymeric materials, particularly polymeric composite materials, have been used to make gas turbine engine and missile components. They also have been used in other applications requiring low weight and high strength. Although many applications to date have involved military products, polymeric materials will increasingly be used in civilian products. Two near term civilian applications are advanced commercial gas turbine engines and the High Speed Civil Transport.
When used for gas turbine engines or other aerospace applications, polymeric materials are often exposed to hot air environments that can cause thermal and thermo-oxidative degradation. For example, parts in a gas turbine engine can be exposed to air at temperatures between 315.degree. C. and 400.degree. C. To be suitable for many applications, polymeric materials must resist oxidation or degradation in flowing air at pressures up to 480 kPa (70 psi) and at temperatures that can exceed 370.degree. C.
Polyimide materials, including PMR-II-30, PMR-II-50, PMR-15, AFR-700B, and Avimid-N.TM., are among the polymeric materials currently used in aerospace applications. PMR-II-30, PMR-II-50, and PMR-15 were developed by NASA Lewis and are licensed to various companies. AFR-700B was developed by TRW, Inc. (Redondo Beach, Calif.) under contract to the United States Air Force. Avimid-N.TM. was developed by E.I. DuPont de Nemours and Company (Wilmington, DE). A particularly promising group of polyimides are 3F-polyimides, also known as 3F-PI. 3F-PI materials include 3F-PDA. 3F-PDA comprises polymer units having the formula: ##STR1## 3F-PI materials are made by polymerizing 4,4'-(2,2,2-trifluoro-1-phenylethylidene) diphthalic anhydride or a dialkylester of 4,4'-(2,2,2-trifluoro-1-phenylethylidene) diphthalic anhydride with a diamine. 3F-PI materials are described in more detail in commonly assigned U.S. Pat. Nos. 4,742,152, 4,801,682, and 4,863,640, all to Scola. Like other current polyimide materials, 3F-PI materials can have acceptable thermo-oxidative stability under some conditions. It would, however, be desirable to improve their thermo-oxidative stability to permit them to function under more severe conditions.
Therefore, what is needed in the industry are 3F-PI resins that have better thermo-oxidative stability than prior art polyimide resins.